Import ANSI C cityhash.

The implementation is the ANSI C adaptation (by Alexander Nusov)
of Google's original C++ implementation.

The whole is covered by an MIT-style license.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Fix (c) indentation.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Use CityHash in AVL dirent cache.

CityHash is a FAST hashing algorithm by Google, based on
murmur3.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
 Use CityHash in DRC.

 CityHash is a FAST hashing algorithm by Google, based on
 murmur3.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>